1. Field of the Invention
The present invention relates to a liquid crystal display device and a method for producing the same. In particular, the present invention relates to a liquid crystal display device in which a liquid crystal region and a polymeric wall surrounding the liquid crystal region are interposed between a pair of substrates, at least one of which is transparent, provided with electrodes, and a method for producing such a liquid crystal display device.
2. Description of the Related Art
At present, liquid crystal display devices of a variety of display modes are used. As a liquid crystal display device utilizing an electro-optic effect, for example, liquid crystal display devices of a twisted nematic (TN) mode or a super-twisted nematic (STN) mode, which use nematic liquid crystal, have been put into practical use. Moreover, a liquid crystal display device utilizing ferroelectric liquid crystal (FLC) has been recently put into practical use, too.
Alternatively, a liquid crystal display device, which utilizes birefringence of liquid crystal so as to electrically control the display to be a transparent state or an opaque state, has been proposed. In such a liquid crystal display device, the refractive index of the liquid crystal molecules with respect to ordinary light is basically matched with the refractive index of a polymer working as the display medium. More specifically, in the liquid crystal display device, the transparent state is shown when the liquid crystal molecules are uniformly aligned in the direction of the electric field by application of a voltage, and the opaque state is shown under application of no voltage because of light scattering caused by the turbulence of the liquid crystal molecules.
As a method for producing such a liquid crystal display device, the following methods have been proposed: (1) a method for providing a display medium by allowing polymer capsules to contain liquid crystal, as disclosed in Japanese National Publication No. 58-501631; (2) a method for forming a liquid crystal region in a mixture of a liquid crystal material and a polymeric material by mixing the polymeric material such as a photocurable resin (or a thermosetting resin) and the liquid crystal material, and then curing the polymeric material by light (or heat) to cause phase separation of the liquid crystal material from the polymeric material, as disclosed in Japanese National Publication No. 61-502128; (3) a method for controlling the diameter of a liquid crystal region in the form of a droplet, as disclosed in Japanese Laid-Open Publication No. 3-72317; (4) a method for impregnating a porous polymeric film with liquid crystal, as disclosed in Japanese Laid-Open Patent Publication No. 3-59515; and (5) a method for floating polymeric beads working as a source of light scattering in liquid crystal interposed between two transparent electrodes which are isolated from each other, as disclosed in Japanese Laid-Open Patent Publication No. 3-46621. These methods, however, have the following problems, respectively.
(1) In the case of providing a display medium by allowing polymer capsules to contain liquid crystal, liquid crystal molecules contained in each polymer capsule form an independent liquid crystal region. For this reason, driving voltage for aligning the liquid crystal molecules is varied with respect to each liquid crystal region. As a result, a high driving voltage is required in order to simultaneously operate all of the liquid crystal regions, thereby limiting the range of use as a liquid crystal display device. PA1 (2) In the case of forming a liquid crystal region in a mixture of a liquid crystal material and a polymeric material, it is difficult to form the liquid crystal region at a desired position in the mixture precisely; PA1 (3) In the case of controlling the diameter of a liquid crystal region in the form of a droplet, it is difficult to control the diameter of the liquid crystal region precisely. PA1 (4) In the case of impregnating a porous polymeric film with liquid crystal, the phase separation is not used for forming a liquid crystal region. Accordingly, the selection of applicable polymeric materials and liquid crystal materials has a wide range, and porous polymeric films impregnated with liquid crystal can be easily formed. On the other hand, currently, this method has a problem in that it is difficult to precisely control the diameter of the liquid crystal region in the form of a droplet and to precisely form the liquid crystal region in the direction along the substrate surface. PA1 (5) In the case of floating polymeric beads working as a source of light scattering in liquid crystal, it is difficult to uniformly disperse the beads, although the intensity of the light scattering is great. Therefore, it is difficult to generate the same extent of scattering with respect to each pixel, resulting in display irregularity.
As described above, in the polymer dispersed type liquid crystal display device using the polymeric liquid crystal and having liquid crystal regions in the form of a droplet dispersed therein, the shape of the liquid crystal regions is not uniform and it is difficult to precisely control locations of the liquid crystal regions in the direction along the substrate surface, because of a production method thereof. In such a polymeric dispersed type liquid crystal display device, a driving voltage is varied with respect to each liquid crystal region, in addition to the failure of the precise control of the locations of the liquid crystal regions. Accordingly, the liquid crystal display device of such a type does not show steep electro-optic characteristics at a threshold value, and the driving voltage is relatively high. Moreover, such a polymer dispersed type liquid crystal display device has another problem in that contrast is relatively low because there exist a great number of liquid crystal regions of low light scattering ability.
Furthermore, in such a liquid crystal display device, it is not possible to enlarge the screen while maintaining a high definition. In addition, in the case of a duty driving system, in which the liquid crystal display device is driven in accordance with values obtained by averaging signals, it is impossible to make the duty ratio high.
In order to solve the above problems in the polymer dispersed type liquid crystal display device, the inventors of the present invention have discovered a novel display mode. This display mode has the following features: (1) phase separation of a liquid crystal material from a polymeric material is more definite, and a liquid crystal region is formed in a pixel area and a polymeric region is formed in a non-pixel area; and (2) the polymeric region is formed in a shape of a column so as to obtain a polymeric wall surrounding the liquid crystal region, acting as a spacer. Thus, a liquid crystal display device having such a display mode is excellent in an impact resistance. As methods for producing a liquid crystal display device having such a mode, the inventors of the present invention have proposed the following two production methods.
A first production method includes the steps of: (1) injecting a mixture of a liquid crystal material, a photocurable resin as a polymeric material and a photopolymerization initiator between a pair of substrates disposed opposite each other; (2) providing a photomask on one of the substrates so as to prevent light from being radiated to pixel areas; and (3) irradiating the mixture with ultraviolet rays (hereinafter referred to as UV rays) over the photomask. Thus, liquid crystal regions are formed in the pixel areas, which are weakly irradiated, while polymeric regions are formed in the non-pixel areas, which are intensely irradiated. Such a production method of a liquid crystal display device is disclosed in Japanese Laid-Open Patent Publication No. 6-301015. According to this production method, a photomask is used so as to prevent light from being radiated to pixel areas, so that liquid crystal regions can be formed in the pixel areas at desired locations.
A second production method uses a self-alignment method in which an indium tin oxide (ITO) electrode is used as a photomask. This production method uses the ITO electrode as a photomask utilizing the characteristic that an ITO electrode absorbs UV rays. The difference between transmittance of ITO electrode portions and that of non-ITO electrode portions is used to form an intensely irradiated region and a weakly irradiated region, thus obtaining liquid crystal regions in the pixel areas.
In the above production methods, the ultimate objective is to cause complete phase separation to such an extent that the photocurable resin does not remain in the pixel areas, i.e., to obtain a liquid crystal region completely separated from a polymeric region. However, it is extremely difficult to cause the complete phase separation of the liquid crystal material from the photocurable resin. In other words, in the above production methods, there are many cases where the photocurable resin is left in the liquid crystal region, or the liquid crystal material remains in the polymeric region (i.e., the polymeric wall). In the case where the photocurable resin is left in the liquid crystal region, the following problems arise: (1) the aperture ratio of the panel is reduced due to the left photocurable resin; or (2) the alignment state of the liquid crystal molecules is deteriorated by the photocurable resin left on the alignment film, so that, the optical characteristics (e.g., response speed or contrast) of the liquid crystal display device are deteriorated. On the other hand, in the case where the liquid crystal material remains in the polymeric wall, the following problems arise: (1) the optical characteristics of the liquid crystal display device are deteriorated by light scattering caused in the non-pixel areas; or (2) the strength of the polymeric wall is not sufficient, so that the impact resistance of the liquid crystal display device is not sufficient.
As described above, when the phase separation of the liquid crystal material from the photocurable resin is not completely conducted, a variety of problems arise. Therefore, a liquid crystal display device in which the liquid crystal material and the photocurable resin are more distinctly phase-separated is desired.